It is previously known to make drills for metal drilling with two indexable inserts of tungsten carbide, where the first insert is a center insert which will cut the central parts of the hole, and the second insert is a peripheral insert which will cut the peripheral parts of the hole. The inserts are substantially located each on one side of the axis of the drill, but a portion of the center insert will often extend past the axis. Some such drills are described in the patents EP 088 505, EP 181 844, EP 292 455, U.S. Pat. No. 4,124,328 and U.S. Pat. No. 4,268,198. Compared to drills with a continuous insert over the whole diameter of the drill, such as U.S. Pat. No. 4,527,931, drills with center and peripheral inserts have the advantage of cutting narrower chips which are more easily conveyed out of the hole, but the disadvantage of large unbalanced lateral cutting forces, tending to bend the drill or displace the work piece resulting in impaired precision. For this reason, drills with center and peripheral inserts have up to now been used for short holes only, and require rigid clamping of the work piece. Since the forward extremity of the center insert is not on the axis of the drill, it has no tendency to center itself on an existing hole of smaller diameter and is not suitable for chamfering.
This is especially disturbing in starting a new hole when the drill is not yet stabilized by the outer edge of the peripheral insert. Unbalanced forces in the plane of the edges will bend the drill to give the hole an incorrect diameter, and forces at right angles to the inserts will make it difficult to start a hole at the point intended, especially on work pieces with small thickness. Unbalanced lateral forces will occur either because the length of actively cutting edge on one side of the drill axis is not equal to that on the other side, or because the axis does not lie in the planes of the edges. The unbalanced forces will be particularly severe if the surface of the work piece is uneven or is not at right angles to the drill axis.